The present invention relates to active noise cancellation systems.
The objective in active noise cancellation is to generate a waveform that inverts a nuisance noise source and suppresses it at selected points in space. In active noise cancelling, a waveform is generated for subtraction, and the subtraction is performed acoustically, rather than electrically.
In a basic active noise cancellation system, a noise source or vibration is measured with a local sensor such as an accelerometer or microphone. The noise propagates acoustically over an acoustic channel to a point in space where noise suppression is desired, and at which is placed another microphone. The objective is to remove the acoustic energy components due to the noise source. The measured noise waveform from the local sensor is input to an adaptive filter, the output of which drives a speaker. The second microphone output at the point to be quieted serves as the error waveform for updating the adaptive filter. The adaptive filter changes its weights as it iterates in time to produce a speaker output that at the microphone looks as much as possible (in the minimum mean squared error sense) like the inverse of the noise at that point in space. Thus, in driving the error waveform to have minimum power, the adaptive filter removes the noise by driving the speaker to invert it.
Many previous active noise cancelers use the filtered-X LMS algorithm, which requires a training mode. The function of the training mode is to learn the transfer functions of the speaker and microphones used in the system so that compensation filters can be inserted in the feedback loop of the LMS algorithm to keep it stable. As the physical situation changes, the training mode must be reinitiated. For example, in an automobile application to suppress noise within a passenger compartment, the training mode may need to be performed again every time a window is opened, or another passenger enters the compartment, or when the automobile heats up during the day. The training mode can be quite objectionable to passengers in the vehicle.
Commonly assigned U.S. Pat. No. 5,117,401, the entire contents of which are incorporated herein by this reference, describes an active adaptive noise canceller which does not require a training mode. The insertion of a time delay in the computation of the weight updates modifies the frequency stability regions of the canceller. Hence, the canceller provides a mechanism through which the adaptive noise cancellation can be easily adapted to suit any application at hand by simply adjusting the time delay value to acquire the desired frequency stability regions.
In a canceller system employing delay in the filter weight updating, as described in U.S. Pat. No. 5,117,401, it is convenient to use delay values which are integer multiples of the digital sampling period. To provide the flexibility to insert relatively small time delays, which will result in a small change in the canceller frequency stability regions, it is necessary to employ relatively high sample rates.
It is therefore an object of the present invention to provide an active noise cancellation system employing an adaptive filter and a delay in the filter weight updating which can be a non-integer multiple of the sample period.